Early Silurian graptolites from southeastern Alaska and their correlation with graptolitic sequences in North America and the Arctic

Abstract

A succession of graptolite faunas across the Ordovician-Silurian boundary has been discovered in North America. The earliest Silurian graptolite zones, characterized by the association of the biserial graptolites Climacograptus, Glyptograptus, and Orthograptus, established much earlier in the Ordovician together with the first Silurian monograptids, Akidograptus and Dimorphograptus are described for the first time in North America. These new collections of Lower Silurian graptolites are from dark shales in southeastern Alaska and Nevada. The occurrence of Silurian graptolites in western North America is reviewed, and the Alaskan graptolite faunas are correlated with other graptolitic successions in North America and the Soviet Arctic. The Alaskan graptolite species and zonal succession are remarkably similar to that in the Birkhill Shales of Scotland, the type section for the standard Lower Silurian graptolite zones. From Alaska, graptolite assemblages equivalent to the zones of Akidograptus acuminatus, Monograptus cyphus, and Monograptus grcgarius are recognized and illustrated. In addition, the following new graptolite species are established: Climacograptus indivisus esquibelensis, C. innotatus obesus, C.